1. Field of the Invention
The present invention relates to a method and apparatus for producing a light transmitting article of synthetic resin which has a gradient refractive index.
2. Description of the Prior Art
Among light transmitting articles, there is known a transparent rod having a refractive index which gradually decreases in proportion with the square of the distance from the axis. Such a rod works like a convex lens, and the distribution of the refractive index in it may be approximated by the equation (1). EQU n(r)=n.sub.o (1-1/2Ar.sup.2) (1)
wherein n(r) is the refractive index at a point of distance r from the axis; n.sub.o is the refractive index at the axis; and A is a positive constant.
Light entering one end of the rod is transmitted to the other end sinusoidally with a period of L which is approximated by the equation (2). L is called the pitch. ##EQU1##
On the other hand, a transparent rod having a refractive index which gradually increases in proportion with the square of the distance from the axis as approximated by the equation (3) works like a concave lens. EQU n(r)=n.sub.o (1+1/2Br.sup.2) (3)
wherein n(r) and n.sub.o are defined as above, and B is a positive constant.
The method for producing a light transmitting article of syntheic resin having a gradient refractive index as mentioned above is disclosed in Japanese Patent Publication No. 5857/1977 and Japanese Patent Laid-open Nos. 16394/1976 and 119939/1979. According to the method disclosed in the first two patents, a liquid monomer Ma capable of forming a network polymer Pa through crosslinking is subjected to initial polymerization to form a transparent body material in the form of gel. The body material is then steeped in another liquid monomer Mb which forms a polymer Pb having a refractive index different from that of the network polymer Pa, so that the monomer diffuses into the body material in such a way that the monomer concentration in the body material gradually decreases toward the inside. The polymerization of the monomer Mb is accomplished simultaneously with diffusion and/or during the heat treatment performed after diffusion. In this step, the polymerization of the monomer Ma is also brought to completion.
The above-mentioned method, however, has some disadvantages resulting from the steeping of the body material in the liquid monomer Mb. In the case where the diffusion and polymerization of the monomer Mb are performed simultaneously, the liquid monomer Mb becomes viscous due to polymerization taking place slowly in the liquid phase because the steeping temperature is kept comparatively high and the polymerization initiator contained in the body material dissolves in the monomer Mb with the lapse of steeping time. The viscous substance thus formed sticks to the surface of the body material when the body material is removed from the liquid monomer after the diffusion process. In the heat treatment process, the body material absorbs the monomer Mb from the viscous substance it carries. This adversely affects the refractive index at the peripheral part of the light transmitting article. This drawback may be overcome by adding a polymerization inhibitor to the monomer Mb so that it does not polymerize in the liquid phase. But this causes another trouble. That is, the polymerization inhibitor diffuses into the body material together with the monomer Mb and it prevents the complete polymerization in the heat treatment process.
In the case where the polymerization is performed after the diffusion of the monomer Mb is complete, the steeping is carried out at a comparatively low temperature but the monomer Mb which has diffused into the body material vaporizes during the heat treatment process. This again adversely affects the refractive index at the peripheral part of the light transmitting article.
The method disclosed in the above-mentioned Japanese Patent Laid-open No. 119939/1979 was developed to improve the above-mentioned methods, thereby avoiding the distorted distribution of the refractive index in the vicinity of the peripheral part. According to this method, the diffusion of the monomer Mb is performed in the gas phase. In other words, the body material is placed in the vapor of the monomer Mb for a prescribed period of time so that the monomer Mb diffuses into the body material. Simultaneously with the diffusion, the monomer Mb is partly polymerized, and it is completely polymerized by subsequent heat treatment. This method has the following advantages.
(1) The monomer Mb does not stick unnecessarily to the surface of the body material. Therefore, the additional diffusion of the monomer Mb into the body material does not take place during the heat treatment. Since the diffusion is performed at a high temperature, the monomer Mb polymerizes and becomes immobilized while it diffuses into the body material. Thus the monomer Mb does not vaporize from the body material during the heat treatment. This leads to the production of a light transmitting article in which the distortion of refractive index at the peripheral part is minimal and the initial distribution of refractive index is broad. PA1 (2) The polymerization of the monomer Mb in the liquid phase can be prevented by adding a polymerization inhibitor to the liquid monomer Mb from which the vapor of the monomer Mb is supplied. If the inhibitor is one which has a low vapor pressure and vaporizes very little, it will not diffuse into the body material; and therefore, the polymerization will not be inhibited in the heat treatment process. Moreover, if the feed rate of the monomer Mb is properly adjusted, it would not be necessary to add any polymerization inhibitor. This permits the monomer Mb to be recycled repeatedly with a high rate of recovery.
In spite of these advantages, the vapor phase diffusion method still has the following drawbacks. The concentration of the monomer Mb in the gas phase depends on the vapor pressure at a given temperature of the gas phase, and the monomers having a high vapor pressure for efficient diffusion are limited in kind. For example, 1,1,3-trihydroperfluoropropyl (monomer Mb) cannot be used for diffusion into a thick body material (4 mm or more in diameter) by the vapor phase diffusion method as disclosed in Japanese Patent Laid-open No. 119939/1979 because of its low vapor pressure, although it can be used in combination with diethylene glycol bisallyl carbonate (monomer Ma) for producing a light transmitting article of synthetic resin having low chromatic aberration according to the method disclosed in Japanese Patent Laid-open No. 16394/1976.
The conventional method was poor in productivity because the process for producing the body material, the process for diffusing the monomer Mb, and the process of heat treatment are all carried out batchwise. The batchwise operation has the following drawback. The composition of the body material fluctuates in the lengthwise direction either when there is a slight variation in temperature and monomer concentration in the apparatus into which the vaporized or atomized monomer Mb is introduced for diffusion into the body material, or when there is a slight variation in temperature distribution in the heat treatment apparatus. The fluctuation of the composition of the body material in length-wise direction results in a light transmitting article which is not uniform in optical performance along its length. For the reasons mentioned above, the conventional method was unable to efficiently provide light transmitting articles of uniform quality.